Transient Radio Emission from SAX J1808.4−3658

Abstract

We report on the detection of radio emission from the accretion-powered X-ray millisecond pulsar SAX J1808.4-3658, using the Australia Telescope Compact Array. We detected an ~0.8 mJy source at the position of SAX J1808.4-3658 on 1998 April 27, approximately 1 day after the onset of a rapid decline in the X-ray flux; no such source was seen on the previous day. We consider this emission to be related to the radio emission from other X-ray binaries, and it is most likely associated with an ejection of material from the system. No radio emission was detected at later epochs, indicating that if SAX J1808.4-3658 is a radio pulsar during X-ray quiescence, then its monochromatic luminosity must be less than L_1.4GHz ~6 mJy kpc^2.